1. Field of the Invention
The present invention relates to an active noise control system for reducing an in-compartment noise caused by a vibratory noise generated by a vibratory noise source on a vehicle with a canceling sound that is in opposite phase with the in-compartment noise.
2. Description of the Related Art
Heretofore, there has been known the technology of an active noise control apparatus for reducing an in-compartment noise at the position of a microphone placed in the passenger compartment of a vehicle, by detecting the in-compartment noise with the microphone and outputting, from a speaker placed in the passenger compartment, a canceling sound that is in opposite phase with the in-compartment noise based on the in-compartment noise and an engine rotation signal which is correlated to the vibratory noise of an engine on the vehicle (see Japanese Laid-Open Patent Publication No. 2006-084532 and Japanese Patent No. 3843082). The active noise control apparatus cancels out a noise (hereinafter also referred to as “engine noise” or “engine muffling sound”) in the passenger compartment which is caused by the vibratory noise of the engine, of the in-compartment noise.
The in-compartment noise also includes, in addition to the engine noise, a noise (hereinafter also referred to as “driveline noise”) in the passenger compartment that is caused by a vibratory noise of a rotating driveline component such as a propeller shaft, a drive shaft, or the like while the vehicle is running. According to Japanese Laid-Open Utility Model Publication No. 62-200034, it has been proposed to provide a torsional damper around a propeller shaft for dampening torsional vibrations of the propeller shaft thereby to reduce the noise generated by the differential.
The noise is generated by the differential because the propeller shaft which is relatively long and heavy is not well balanced upon rotation. The torsional damper disposed around the propeller shaft for reducing the noise makes the vehicle heavy as a whole and also makes the vehicle costly to manufacture. Alternatively, instead of the torsional damper, weights may be added to vibration-causing regions of the driveline, or production-induced variations of the components of the driveline may be strictly controlled, to reduce the driveline noise. These countermeasures, however, are still liable to make the vehicle heavy as a whole and also to make the vehicle costly to manufacture.
Attempts have been made to reduce the driveline noise with the active noise control apparatus described above. However, since the active noise control apparatus is based on the fact that the engine noise is generated in synchronism with the rotation of the output shaft of the engine, and generates the canceling sound using the frequency of the engine rotation signal depending on the rotational speed of the output shaft, the active noise control apparatus cannot directly be applied to reduce the driveline noise.
This is because the engine is occasionally disconnected from a transmission by a lockup control function of an automatic transmission vehicle or a clutch function of a manual transmission vehicle, making it difficult to calculate the rotational speed and rotation frequency of a driveline component such as a drive shaft, a propeller shaft, or the like at all times from the rotational speed of the output shaft of the engine. Therefore, even if the canceling sound is generated using the frequency of the engine rotation signal, it is difficult to reduce the noise in the passenger compartment (driveline noise) due to the vibratory noise of the driveline.